Notification of or detecting screenshot before being taken?

Is there a notification or other mechanism of being informed that the us开发者_StackOverflower is taking a screenshot with the home/power buttons?

I've seen threads about wanting to disable the taking of screenshots, but that's not what I'm looking to do.

I have a photographer client who's concerned that his works will be copied by means of users taking screenshots and I thought that if there was an opportunity to put a watermark across the image before the screenshot was taken, that would allay his fears.

The PictureWasTakenNotification Darwin notification will be sent when the user takes a screenshot. However, this is sent after the screenshot is taken.

(No notifications will be sent before the screenshot was taken.)

Here's a way which might work, although it will totally go against user interface guidelines I'm sure. If you force the user to have their finger on the screen for the image to show then I don't think they can create screenshots. Because as soon as you press the home+lock keys to actually take the screenshot, the screen seems to behave as if there are no fingers touching it. Try taking a screenshot while moving between home screens to see what I mean.

Not a perfect solution by any means but you may be able to work it into your app design if you're really clever without it detracting too much from the user experience (a tough challenge though!). Nevertheless, I believe this may allow you to display artwork/photos without allowing users to take screenshots.

Since iOS 7 the UIApplicationUserDidTakeScreenshotNotification exists. So doing something like this should detect the screenshots:

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(userDidTakeScreenshot) name:UIApplicationUserDidTakeScreenshotNotification object:nil];
}

- (void)userDidTakeScreenshot {
    // Screenshot taken, act accordingly.
}

Finally, don't forget to remove the observer:

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationUserDidTakeScreenshotNotification object:nil];
}

What's really needed is a notification that is sent before the actual screen capture happens. A delegate method or some other means of giving the app a screenshotting-in-flight opportunity to redraw your content before the grab happens.

And there isn't one.